1. Field of the Invention
The present invention relates to an automobile antenna system and, more particularly, to an improved automobile antenna system for effectively detecting broadcast radio waves received by the vehicle body and then transferring detected signals to various receivers located in a vehicle.
b 2. Description of the Prior Art
Antenna systems are indispensable to modern automobiles which must positively receive various broadcast waves for radio, television and telephone, at receivers located in the vehicle compartment. Such antenna systems are very important also for citizen band transceivers.
One of the conventional antenna systems is known as a pole-type antenna which projects outwardly from the vehicle body of an automobile. Although the pole antenna is superior in performance in its own way, it becomes an obstacle in vehicle body styling.
Furthermore, the pole antenna is disadvantageous in that it is subject to damage, mischief or theft and also in that the antenna becomes a cause of noises produced during high-speed driving. For this reason, there has heretofore been a strong demand for eliminating the pole antenna.
In recent years, there has been increased the number of frequency bands for broadcast or communication waves received at automobiles. A plurality of pole antennas are required according to the increased number of frequency bands. This raises other problems in that the plurality of pole antennas may damage the aesthetic appearance of the automobile and in that reception performance may be deteriorated by electrical interference between the antennas.
Efforts have been made to eliminate the pole antenna system or to conceal the same behind the vehicle body. One of the proposals is that a length of antenna wire is applied to the rearwindow glass of an automobile.
Another proposal is that one utilizes surface currents induced by broadcast waves on the vehicle body of an automobile. This apparently provides the most positive and efficient means for receiving broadcast waves. However, experiments show that such a proposal does not provide any satisfactory results.
One of the reasons why surface currents induced by broadcast waves have not been utilized well is that their induced value is not as large as expected. The prior art mainly used surface currents induced in the roof panel of the vehicle body. In spite of this, surface currents a satisfactory level have not been obtained.
Another reason is that surface currents contain very high level noises. Such noises are mainly generated from the engine ignition system and the battery charging regulator and cannot be eliminated unless the engine is stopped. Noises migrating into the vehicle compartment make it impossible to effect any practicably clear reception of broadcast waves.
In such a situation, some proposals have been made to overcome the above problems. One such proposal is disclosed in Japanese Patent Publication Sho No. 53-22418 in which electrical insulation is formed at a portion of the vehicle body on which currents are concentrated, with the currents being detected directly by a sensor between the opposite ends of the insulation. Although such a construction can detect practicable signals which are superior in S/N ratio, a pickup used therein requires a particular cutout in the vehicle body. This cannot be accepted in the mass-production of automobiles.
Another proposal is shown by Japanese Utility Model Publication Sho No. 53-34826 in which an antenna including a pickup coil for detecting currents in the pillar of a vehicle body is provided. This is advantageous in that the antenna can completely be disposed behind the vehicle body. However, it is not practical that the pickup coil used therein must be located adjacent to the vehicle pillar in a direction perpendicular to the longitudinal axis of the pillar. It also appears that such pickup arrangement cannot obtain any practicable output of the antenna.
As has been described above, the conventional antenna systems are not successful in efficiently detecting currents induced in the vehicle body by broadcast waves.
No effective measure has heretofore been proposed for overcoming the above-described conventional principal problems of providing, particularly, a pickup structure for effectively detecting currents induced in the vehicle body by broadcast waves and a pickup arrangement for obtaining a practicable S/N ratio. The results of various kinds of experiments showed that it might be basically impracticable to use an antenna system utilizing vehicle body currents.